Recently, the pressure sensitive cholesteric liquid crystal (ChLC) writing tablet, Improv Electronics® Boogie Board® LCD eWriter, has appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the tablet as described in U.S. Pat. No. 6,104,448, which is incorporated herein by reference in its entirety. (Improv Electronics® is a unit of Kent Displays, Inc.) This tablet offers a considerable improvement over previous tablet technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrodes in the tablet. In a ChLC writing tablet, the liquid crystal is sandwiched between two substrates, each having an electrode, which are spaced to a particular gap. The upper substrate is flexible and the bottom substrate is painted with an opaque light absorbing background. Within the gap is a bistable ChLC dispersed in a polymer. This material can exhibit two textures, an essentially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The tablet is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the generally transparent texture. When one presses on the top substrate with a point stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from generally transparent to a brilliant reflective color at the location of the stylus. The reflective color contrasts well with the dark background of the lower substrate. An image traced by the stylus or finger will remain on the tablet indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to the transparent conducting electrodes on the inner surface of the substrates that drive the ChLC from its color reflective state back to its generally transparent state.
The above described principle is disclosed in more detail in U.S. Pat. No. 6,104,448, which is incorporated herein by reference in its entirety. Polymer dispersions can be used to control the pressure sensitivity and resolution of the image as described in U.S. Pat. No. 8,228,301, which is incorporated herein by reference in its entirety. Other modes of operation and a tablet for multiple color images are described in this patent and a means for select erase is disclosed in U.S. Pat. No. 8,139,039, which is incorporated herein by reference in its entirety. One mode of operation different from that described above is one in which the tablet is initialized by electrically driving the tablet display to the color reflective texture with a voltage pulse or pulses. Then with a continuous voltage applied to the electrodes of an appropriate value, one can write images by driving the cholesteric material to the generally transparent texture with the pressure of a pointed stylus. This mode of operation with a color reflective background is termed “Mode A” and the other mode with a generally light absorbing dark background is termed “Mode B.” Mixed modes are also possible.
The commercial Boogie Board® writing tablet, operated in Mode B, has the color black for the fixed opaque light absorbing background. The dark black background offers high contrast for the color reflective image written on the tablet. As disclosed in U.S. Pat. No. 5,493,430, which is incorporated herein by reference in its entirety, other opaque colors may also be used for the fixed background of a ChLC display. The color of the background additively mixes with the reflective color to present a different color than that of the ChLC. There may be multiple colors on the background and those colors may be patterned. As an example, the pattern could be lines offering a lined tablet for convenience in writing text similar to a ruled paper tablet.
The Boogie Board® tablet contains the electronics used to erase the tablet. There are other devices such as Liquid Crystal Paper (see U.S. patent application Ser. No. 13/621,367, which is incorporated herein by reference in its entirety) that possess no such electronics. The paper is erased by removably attaching it to external electronic circuitry when it is desired to erase images that have been drawn on its surface. Like normal writing paper, liquid crystal paper is a thin, flexible sheet. One uses liquid crystal paper in the same way normal writing paper is used for handwriting or drawing pictures. An untethered, pointed stylus is used to write on the liquid crystal paper but with the advantage that it does not have to be a pencil or pen but only a pointed object which could even be one's fingernail. For example, the stylus could be comprised of polymer and contain no lead or ink. Like normal writing paper, liquid crystal paper is placed on a hard surface for writing. Erasing liquid crystal paper requires that temporary electrical contact is made with an external circuit in order to provide voltage pulses suitable for clearing it. This might be inconvenient to the user. Likewise only one sheet can be erased at a time.
What would be desired but has not yet even been contemplated to our knowledge, would be a device that would, upon command, erase the liquid crystal paper or ChLC writing tablet at a distance, or in an otherwise electrically isolated manner without direct electrical contact of the electrodes to an erasing circuit. Such a device would not require picking up the paper and connecting it to the erasing device. Further, more than one sheet could be erased at a time. We disclose here such a device.